1. Field of the Invention
The present invention relates to a boot covered by a constant velocity joint that is indispensable for a joint for a drive shaft of a front-wheel-drive vehicle for protecting a joint portion of the constant velocity joint from invasion of water or dust.
2. Related Art
Hitherto, a joint portion of a constant velocity joint is covered by an accordion-folded boot and thus smooth rotation thereof at large angle is maintained by preventing invasion of water or dust. The constant velocity joint boot is constructed of a larger cylindrical portion of a large diameter to be held by a mating member such as a joint outer lace, a smaller cylindrical portion of a smaller diameter than the larger cylindrical portion held by a shaft, and a bellows portion in the shape of substantially truncated cone for integrally connecting the larger cylindrical portion and the smaller cylindrical portion. When in use, since the bellows portion is deformed in accordance with variations in angle formed between the mating member and the shaft, even when an operating angle increases, the joint portion can be reliably sealed by the boot.
The outer peripheral surface of the joint outer lace and the like as mating member is not only circular in cross section, but may have a non-circular cross section. When fastened by such a mating member, it is necessary to form the inner peripheral surface of the larger cylindrical portion of the boot into a non-circular shape corresponding to the outer peripheral surface of the mating member. However, the cross-section of the outer peripheral surface of the larger cylindrical portion is a circular form because it must be a fastened by a ring-shaped fastening member. Therefore, the larger cylindrical portion must consequently be formed with a thick-walled portion and a thin-walled portion.
It is convenient to manufacture the boot by blow molding. However, it is difficult to form the larger cylindrical portion constituting of the thick-walled portion and the thin-walled portion by blow molding. Therefore, forming the larger cylindrical portion into a bilayer structure, a process of forming an upper layer on the outer peripheral side with the bellows portion by blow molding, and integrating them by inserting a bush formed separately by injection molding into the upper layer is performed. However, in this case, two forming processes are required, which is disadvantageous in that a large number of processes are required.
Therefore, in recent years, a process of integrally forming the larger cylindrical portion constructed of the thick-walled portion and the thin-walled portion, the bellows portion and the smaller cylindrical portion by injection blow molding is performed. However, when forming the larger cylindrical portion constructed of the thick-walled portion and the thin-walled portion by injection molding, there may be the case in which surface sink is generated at the thick-walled portion after molding, and consequently, a gap is formed with respect to the mating member when fastened. Consequently, it may result in a defective good, which may lead to leakage of grease.
In order to improve such defect, it is effective to form the thick-walled portion into a hollow shape, and reduce the solid portion. Therefore, for example, in EP 0915 264 A2, there is disclosed a boot having a plurality of ribs on the larger cylindrical portion in the direction of thickness and a thick-walled portion being hollow at portions between the ribs. However, in the boot of this kind, there is a problem in that the strength of the rib is tend to be insufficient, and thus sealing property required for deforming the ribs when being fasten by the fastening member cannot be obtained.
In addition in JP-10-110738, a boot having a larger cylindrical portion formed with a slit at the end surface of the thick-walled portion is disclosed. However, in the boot of this kind, since the thick-walled portion is deformed so that the width of the slit is narrowed when being fastened by the fastening member and thus defection is constantly generated, further deflection may occur due to bending of the bellows portion when in use, and thus it may suffer some cracks. There is also a case in which a gap is generated between the wall surface on both sides of the slit. In such a case, an uniform fastening force cannot be obtained and thus required sealing property cannot be obtained.
In JP-A-2002-122237, a boot formed with a slit opening at the outer peripheral surface of the thick-walled portion of the larger cylindrical portion is disclosed. However, in this boot, there is a problem in that stress varying points are generated at both ends of the slit when fastened by the fastening member and thus uniform fastening force cannot be obtained, whereby required sealing property cannot be achieved.